1. Field of the Invention
The invention relates to semiconductor circuit design, and more particularly to modeling temperatures of a self-heating semiconductor device.
2. Background Description
Accurate measurement of self-heating of SOI and SiGe based MOSFET devices is important because DC currents of such devices are typically depressed significantly due to self-heating. This is in contrast to CMOS circuits where transients are too rapid for significant self-heating to occur. Thus, simulation (compact) models must be adjusted to correctly account for self-heating in order to correctly predict circuit performance. In SOI technologies, this effect ranges from about 3 percent to 12 percent, while in SGOI (SiGe on SOI) these effects are expected to exceed 30 percent.
Such large temperature effects in SGOI devices are due in part to the active region of the device being almost entirely surrounded by layers of material having poor thermal conductivity properties. For example, the active region of the SGOI device is SiGe, and the SiGe is arranged on top of an oxide layer. The SiGe layer has a limited length and width on top of the oxide layer, and thus forms what is referred to as a “island” on the oxide layer. During subsequent fabrication steps, the SiGe island is surrounded on its sides by an oxide, and then further covered over its top by an oxide. Thus, the SiGe island is relatively small and almost entirely surrounded by an oxide. Due to the surrounding oxide, the SiGe island has extremely limited thermal pathways by which to dissipate any heat generated in the SiGe island.
The small dimensions of the SiGe island also increase the device's susceptibility to thermal effects. In particular, because the SiGe island is relatively small, it has a comparatively low thermal mass. With the low thermal mass, the SiGe island quickly responds to any heating by a device thereon. As such, the SiGe island itself fails to act as its own heat sink for the device and the device quickly heats the island up to the devices own temperature. Thus, any device fabricated on the SiGe island will be particularly influenced by its own self-heating effects.
Known methods of measuring semiconductor device performance versus temperature include placing a diode proximate to the device for which the temperature will be measured, and using the diode's change in electrical performance as a function of temperature to measure the temperature at that point. However, such a method is difficult to implement because it is difficult to build such a diode close to a device to be measured to provide an accurate gauge of the active region of the device.
Another method of measuring the temperature effect on the electrical characteristics of a semiconductor device includes running the device at a particular power level to heat itself, and using the device's own change in electrical characteristics as a function of temperature to determine the temperature of the device. While simple to fabricate such a temperature measurement configuration, the data produced by such a configuration is less than reliable because of various hysteresis-like effects. For example, the device's sensitivity to temperature changes, may be based on, for example, among other things, on the prior electrical history of the device. Such a sensitivity to electrical history makes determining the actual temperature of the device to be less than reliable.
The invention is designed to solve one or more of the above-mentioned problems.